One of the major problems that radio communication systems face when providing wide area two-way data and/or voice communication coverage is how to allow for a communication unit operating in the system to roam freely between sub-systems which form the wide area system. For example, a 10 site simulcast radio frequency trunked system may be considered a wide area two-way system on a regional level. The problem with such systems is how to provide a way of allowing the communication unit which is part of the system the ability to roam freely so that it is always part of a larger composite wide area system (or communication network), thereby allowing a caller to be able to communicate with the communication unit at any time the communication unit is in range of the wide area system.
Present day solutions to the above problem include communication systems such as Coverage Plus.TM. manufactured by Motorola, Inc. This system uses a registration process in which the roaming communications units use a "checking in" process on each communication network site as the communication unit moves from site to site. This "check-in" process occurs even if the communication unit is not communicating with other units.
In wide area systems, knowing the location of each of the communication units "ahead of time", before a call request that will involve these units occurs, helps speed up group calls that involve activating channel resources at many sites across the wide area system. Present day registration processes take a lot of time on the site control channel which has the effect of limiting the overall capacity of the local site. The overall effect on system capacity will depend on the number of roaming units and the frequency of operation the units are employing when moving from coverage area to coverage area (i.e., site to site registration "hand-offs"). Currently this is not a major problem with the current numbers of roaming units and the relatively large size of the coverage areas, since typically larger coverage areas see fewer hand-offs. However, as systems become more complex, and coverage areas become smaller in size, system overhead delays will become a major problem for wide area systems.
In wide area trunked systems, communication units must find a site within their present coverage area by either scanning through a list of pre-programmed control channels or they must scan the entire trunking band for a "friendly" control channel that is broadcasting a "networking offered here" message. In other words, the communication unit must search for a control channel which is compatible with the communication unit. As one can quickly determine, acquiring access to a local site, while roaming into unknown coverage areas, in the manner discussed above, is extremely time consuming.
In comparison to wide area trunked systems, cellular telephone systems have relatively small cells and large numbers of communication unit users that only do individual oriented calls (i.e., mobile-to-land and land-to-mobile telephone calls). Given this, cellular systems do not use unit-to-site registration processes. Due to the relative low probability of an infrequent incoming land-to-mobile call, it is better for cellular systems to just send a unit respond signal over the control channel from every cell when incoming calls actually occur. Speed of call set ups is not of utmost criticality for these one unit phone calls. Therefore, the unit respond signals can go out constantly, over a period of several seconds, in order to determine if the communication unit is turned on and in communication range. Each cellular radio must scan the different system control channels as the unit moves from cell to cell.
In nation-wide satellite paging systems, although providing wide area coverage, these systems do not direct roaming communication units to local two-way systems. Also, these paging systems do not provide the pagers with local system availability information in order for the pagers to receive further information once they have been paged.
System to system roaming, like that implemented by Ameritech Inc. in their "Easy Roaming" scheme, uses a roaming unit-to-system registration process as the communication unit enters a non-home system. In this system, the registration process sends a message back to the communication unit's home system so it knows where to forward future calls addressed to the unit, should someone try to reach the roaming unit at its normal home system number. One problem with this roaming arrangement is that calls can only be completed if the unit-to-location information is constantly known. This "back to the home system" registration process suffers when communication units are not able to reach the home system (e.g., RF noise, traffic, etc.). This type of registration process also tends to add a lot of control traffic overhead that may be unnecessary to the wide area system.
More importantly than all of the above mentioned problems is that cellular or trunked communication systems are not presently located everywhere, in order to allow communication units the freedom to roam over wide coverage areas. Presently, communication units are not constructed such that they can use either system type interchangeably. For example, today's cellular radios can't switch over to trunking when a cellular system is not available and a trunked system is, or vice versa. The problem has not been limited by the technology of a radio that has sufficient RF bandwidth to cover both services. The problem arises in that the roaming communication units can't decide which system to move to, even if they could, since the communication units do not know how to find these different types of systems quickly enough and they also do not know which one of the communication systems is "best" for their particular service needs at any given point in time (e.g., data, voice, etc.). This scenario will only worsen as newer small cell systems and other different types of communication systems emerge.
Some more specific systems which deal with communication links establishment in communication systems include systems such as TRACKNET.TM. automatic vehicle location system (AVL) manufactured by Motorola, Inc. This AVL system is a mobile to fixed end information gathering system that uses the existing network of U.S. Government Loran-C transmitters to provide vehicle location information. This system can poll units operating in the system via a single mode dedicated radio network. The communication unit responds on this same network with its location. While the Loran-C information is quite accurate, such resolution capability is usually not required when a unit roams outside of its designated home system in many communication system applications. In a lot of communication system applications it is enough to know the approximate vicinity the communication device is located. The problems with such a system is that it requires that the communication devices operating in the system have Loran-C receive capability, as well as being able to receive normal system traffic on another totally different frequency band. Such a requirement is very impractical for portable radios and other smaller communication devices. The need for Loran-C capability also adds a substantial amount of cost to the individual communication devices operating in wide area systems.
In a more specific communication system application, American Telephone and Telegraph Inc., and other corporations are now offering a facsimile (FAX) mail service that operates like the more popular voice mail services which are known in the art. In these systems, the caller dials into the FAX mail system, presumably it would have the same number as the number of the unit being called. If the target unit is roaming, instead of printing the FAX locally, the FAX mail system saves the FAX electronically. When the target unit checks in, the target unit dials the FAX mail system and discovers that there's a FAX pending. The unit then switches to a FAX mode and gets its FAX out of the system. The problem with such a system is that the target unit must still access the FAX machine to receive its FAX. Also, if the target unit is away from its designated home system, the unit will not be able to know that a FAX has arrived for it until it returns to its home system.
As can be seen from the above discussion of prior art systems, a need exists for a way of controlling communication units which operate in communication systems which encompass more than one communication system. A need exists for a wide area system which can provide for improved communication services such as mobile-to-mobile, land-to-mobile, land-to-mobile FAX, land-to-mobile location determination, and other related system features.